The present invention relates to a method for controlling an SCR system having two metering valves, wherein a reducing agent solution is metered in simultaneously via the two metering valves. Furthermore, the invention relates to a computer program which carries out each step of the method if it is running on a computer, and to a machine-readable storage medium which stores the computer program. Finally, the invention relates to an electronic control unit which is designed to carry out the method according to the invention.
Nowadays, the SCR method (Selective Catalytic Reduction) is used in the aftertreatment of exhaust gases from an internal combustion engine in order to reduce nitrogen oxides (NOx) in the exhaust gas. DE 103 46 220 A1 describes the basic principle. Here, a 32.5% aqueous urea solution (AUS), also known commercially as AdBlue®, is metered into the oxygen-rich exhaust gas. Typically, a metering valve within a metering module is provided for this purpose in order to meter the AUS into the exhaust gas flow upstream of an SCR catalytic converter. In the SCR catalytic converter, the AUS reacts to form ammonia, which then combines with the nitrogen oxides, forming water and nitrogen. The AUS is fed to the metering module from a reducing agent tank, through a pressure line, by means of a feed module comprising a feed pump.
SCR systems are furthermore known in which a plurality of SCR catalytic converters is arranged in the exhaust line, acting in combination on the exhaust gas. In this case, the exhaust line can furthermore be branched upstream of the SCR catalytic converters. The AUS metering operations then take place either via a common metering valve, which is arranged upstream of the two SCR catalytic converters, or via a plurality of metering valves, wherein each SCR catalytic converter is assigned a metering valve positioned upstream. By means of the latter, the desired mass of reducing agent solution can be metered in accurately. Typically, the metering valves are connected to a common feed module, wherein the metering valves share at least part of a common pressure line.